BIOCHEMISTRY

ENTRY OF OTHER CARBOHYDRATES

INTO GLYCOLYSIS

DR SAMEER FATANI

ENTRY OF FRUCTOSE, GALACTOSE, AND MANNOSE

INTO GLYCOLYSIS

In most organisms hexoses other than glucose can undergo glycolysis after conversion to a phosphorylated derivatives.

D-fructose. In muscle and kidney:D-fructose, present in many free form in many fruits and formed by hydrolysis of sucrose in small intestine vertebrates, is phosphorylated by HEXOKINASE:

Fructose + ATP Mg2+ fructose 6-phosphate + ADP hexokinase

In the liver:Fructose enters glycolysis by different pathway. The liver enzyme fructokinase catalyzes the phosphorylation of fructose at C-1 rather than C-6:

Fructose + ATP Mg2+ fructose 1-phosphate + ADP fructokinase

Then Fructose 1-phosphate fructose 1-phosphate aldolase Dihydroxyacetone phosphate + Glyceraldehyde

Dihydroxyacetone phosphate triose phosphate isomerse

glyceraldehyde 3-phosphate (intermediate in glycolysis)

And

Glyceraldehyde + ATP Mg2+ glyceraldehyde 3-phosphate + ADP triose kinase

Thus both products of fructose 1-phosphate hydrolysis

enter the glycolytic pathway as glyceraldehyde 3-phosphate.

D-galactose A product of hydrolysis of the disaccharide lactose (milk sugar), passes in the blood from the intestine to the liver, where it is first phosphorylated at C-1 in the presence of ATP, by the enzyme galactokinase:

Galactose + ATP Mg2+ galactose 1-phosphate + ADP galactokinase

The galactose 1-phosphate is then converted to its epimer at C-4, glucose 1-phosphate, by a set of reactions in which uridne diphosphate (UDP) functions as a coenzyme-like carrier of hexose groups. The epimerization involves first the oxidation of the C-4 –OH group to a ketone, then reduction of the ketone to an –OH, with inversion of the configuration at C-4. NAD is the cofactor for both the oxidation and the reduction.

Defects in any of the three enzymes in this pathway cause galactosemia in humans.galactokinase-dificinecy galactosemia: characterized by high galactose concentrations are found in blood and urine. (for rest back to book, L4-p537).

D-mannose Released in the digestion of various polysaccharides and glycoproteins of foods, can be phosphorylated at C-6 b hexokinase:

Mannose + ATP Mg2+ mannose 6-phosphate + ADP

hexokinase

Mannose 6-phosphate is isomerized by phospho-mannose isomeraze to yield fructose 6-phosphate, an intermediate of glycolysis.

Mannose 6-phosphate isomerization fructose 6-phosphate phospho-mannose isomeraze